Internal combustions engines convert chemical energy from a fuel into mechanical energy. The fuel may be petroleum-based (gasoline or diesel), natural gas, a combination thereof, or the like. Some internal combustion engines, such as gasoline engines, inject an air-fuel mixture into one or more cylinders for ignition by a spark from a spark plug or the like. Other internal combustion engines, such as diesel engines, compress air in the cylinder and then inject fuel into the cylinder for the compressed air to ignite. An internal combustion engine may use a camshaft system, a hydraulically activated electronically controlled unit injection (HEUI) system, or the like to control the fuel injection into the cylinders. In each cylinder, the ignited fuel generates rapidly expanding gases that actuate a piston in the cylinder. The piston usually is connected to a crankshaft or similar device for converting the reciprocating motion of the piston into rotational motion. The rotational motion from the crankshaft may be used to propel a vehicle, operate a pump or an electrical generator, or perform other work. The vehicle may be a truck, an automobile, a boat, or the like.
Most internal combustion engines have a cooling system to circulate coolant through the engine. The coolant removes heat from the engine during operation. The coolant may be water, an antifreeze fluid such as ethylene glycol, a combination thereof, or the like. The cooling system usually is connected to a radiator or other heat exchanger that removes heat from the coolant. The cooling system typically has a water or coolant pump that moves coolant through the engine crankcase, around each cylinder, and into the cylinder head. The coolant may flow from the crankcase, through other components in the engine such as an oil cooler, and into the cylinder head. The coolant flows from the cylinder head, through the radiator, and returns to the coolant pump for continued circulation through the engine. The cooling system may have a thermostat to prevent coolant flow through the radiator when the engine is cold such as during engine startup.
Many internal combustion engines use an exhaust gas recirculation (EGR) system to reduce the production of nitrogen oxides (NOx) during the combustion process in the cylinders. EGR systems typically divert a portion of the exhaust gases exiting the cylinders for mixing with intake air. The exhaust gas generally lowers the combustion temperature of the fuel below the temperature where nitrogen combines with oxygen to form nitrogen oxides (NOx).
Many EGR systems have an EGR cooler or heat exchanger that reduces the temperature of the exhaust gases. Generally, more exhaust gas can be mixed with the intake air when the exhaust gas temperature is lower. Additional exhaust gases in the intake air may further reduce the amount of NOx produced by the engine.
Most EGR coolers have a counter flow arrangement to remove heat from the exhaust gases. In the EGR cooler, the exhaust gases pass in one direction along one side of a wall or other barrier. A cooling medium passes in the opposite direction on the opposite side of the wall. The cooling medium may be air, water, or another fluid. When the cooling medium has a lower temperature than the exhaust gases, heat transfers from the exhaust gases through the wall into the cooling medium. The heat transfer lowers the temperature of the exhaust gases. The heat transfer can be increased by increasing the temperature difference between the exhaust gases and the cooling medium. Conversely, the heat transfer can be decreased by decreasing the temperature difference. The heat transfer can be increased by increasing the surface area or length of the wall separating the exhaust gases and the cooling medium. Conversely, the heat transfer can be decreased by decreasing the surface are or length of the wall.
Many EGR coolers use coolant from the engine's cooling system to reduce the temperature of the exhaust gases. Typically, the EGR cooler is connected to another engine component in series so that the same coolant flows through the other component and then the EGR cooler in sequence. In some internal combustion engines, the coolant flows sequentially from the coolant pump through the crankcase, through an oil cooler prior, and then through the EGR cooler. The coolant usually flows from the EGR cooler into the cylinder head, where it combines with coolant from the crankcase for return to the coolant pump.
The sequential flow of coolant through engine components may increase the coolant temperature before the coolant flows through the EGR cooler. In some internal combustion engines, the temperature of coolant into the EGR cooler may be about 3 to 5 degrees higher than the temperature of coolant exiting the coolant pump. The coolant temperature may increase about 1 to 2 degrees as the coolant flows from the coolant pump through the crankcase to the oil cooler. The coolant temperature may increase about 2 to 3 degrees as the coolant flows through the oil cooler to the EGR cooler. These and other internal combustion engines may have different temperature increases as coolant flows through engine components to the EGR cooler.
The higher coolant temperature reduces the heat transfer of the EGR cooler. The lower heat transfer decreases the temperature reduction of the exhaust gases through the EGR cooler. A larger EGR cooler may be needed to provide sufficient heat transfer for a desired exhaust gas temperature. A larger EGR cooler may increase the costs of the EGR and cooling systems. Some engines may not be able to use a larger EGR cooler due to space limitations. These engines may have less exhaust gas recirculation, which may result in lower NOx reduction.